Final answer:
Diffraction is the term used when light from one area spills over into an adjoining area. It occurs when light encounters an obstacle and spreads and bends around it.
Step-by-step explanation:
When light from one area spills over into an adjoining area, it is called diffraction. Diffraction occurs when light encounters an obstacle, such as an edge or a small aperture, causing it to spread and bend around the obstacle. This results in the light extending into areas that would have been in shadow if there was no diffraction.